1. Field of the Invention
The present invention relates to nonaqueous secondary batteries. More particularly it relates to a nonaqueous secondary battery using a carbon material as an anode.
2. Description of the Related Art
Along with the reduction in the size of electronic apparatus and conservation of electric power, a stronger demand is made on creating a secondary battery of an alkali metal type having a high energy density in which an alkali metal such as lithium or the like is used. However, when an alkali metal is used in an anode, batteries using a lithium metal, for example, there is a drawback in that a repetition of charging and discharging cycles produces a dendrite on the lithium metal thereby causing an internal short circuit in batteries. On the other hand, a lithium/aluminum alloy has been proposed as a substitute for the lithium metal. Use of the alloy suppresses the generation of a dendrite. However such batteries using the alloy as an anode have a short cycle life in the deep discharge depth. Thus no essential improvement is made. Consequently attention focussed on a carbon material excellent in cycle properties and safety, which material permits lithium to be intercalated and deintercalated in the form of an ion, in particular, a certain kind of carbon material in which lithium is intercalated to form an interlayer compound is accompanied by an electrochemical reaction such as intercalation and deintercalation of lithium ions in an organic electrolyte including a lithium salt thus enabling a reversible oxidation-reduction reaction. Carbon materials are quite promising for use as an anode of lithium secondary batteries. Thus intense study has been made of secondary batteries using carbon materials.
Carbon has various forms because graphite-like planes are expanded in two dimensions and stacked in various ways. Thus it is possible to obtain various carbons depending on starting materials and production methods. Carbons can be classified into several groups from the viewpoint of orientation state or fine structure thereof. They include random orientation microtexture in which graphite-like layers are stacked at random, a planar orientation microtexture in which graphite-like layers are oriented along a reference plane, an axial orientation microtexture in which graphite-like layers are oriented along an axis (including coaxial cylindrical structures in which graphite-like layers are cylindrically oriented relative to a reference axis, and radial structure in which graphite-like layers are radially oriented relative to a reference axis), and a point orientation microtexture in which graphite-like layers are stacked around a reference point (including concentric structures in which graphite-like layers are spherically oriented relative to a reference point though not in a complete texture, a radial structure in which graphite-like layers are radially oriented relative to a reference point). It is known that carbon having the same interlayer spacing has a different function owing to differences in arrangement of graphite-like layers.
When carbon is used as an anode active material, the quantity of lithium inserted between carbon layers is one lithium atom relative to six carbon atoms, namely C.sub.6 Li at most. Thus the theoretical capacity of carbon per unit weight is 372 mAh/g.
Carbon materials conventionally used as an anode are disclosed in Japanese Laid-Open Patent No. SHO. 62-90863, Japanese Laid-Open Patent No. SHO. 62-122066, Japanese Laid-Open Patent No. SHO 63-213267, Japanese Laid-Open Patent No. HEI. 1-204361, Japanese Laid-Open Patent No.HEI. 2-82466, Japanese Laid-Open Patent No. HEI. 3-252053, Japanese Laid-Open Patent No. HEI. 3-285273 and Japanese Laid-Open Patent No. HEI. 3-289068. The carbon material disclosed in these patent publications does not exhibit a sufficient capacity in the potential range in that can be used as an actual battery, because of a linear increase in potential during the deintercalation of lithium, even if the carbon material has a certain capacity as seen from cokes used as a electrode material. When an electrode is manufactured by using a carbon material, bulk density is an important factor although a real density is also required. Since the shape and size of carbon particles provide the bulk density, it is difficult to raise the capacity density per unit volume with a fibrous carbon as shown in the embodiment of Japanese Laid-Open Patent No. SHO. 62-90863, Japanese Laid-Open Patent No. HEI. 2-82466, Japanese Laid-Open Patent No. HEI. 285273, and Japanese Laid-Open Patent No. HEI. 3-289068. On the other hand, a pyrolytic carbon prepared by the CVD technique as disclosed in Japanese Laid-Open Patent No. SHO. 63-24555 exhibits a high charge and discharge stability. It is, however, difficult to make a thick film electrode and to obtain a large capacity electrode with that method. Then as shown in Japanese Laid-Open Patent No. HEI. 4-296448, deposited carbon is stripped and ground into powders. Such powders are thought to provide a thick film in film forming processes. However, these powders are not suitable for use in such processes, because the carbon is flaky and stripping deposited carbon is troublesome in production.
Furthermore, as seen in Japanese Laid-Open Patent No. 63-230512, a powdered graphite cannot provide sufficient capacity as an active material of batteries because no orientation graphite-like layer is observed. Furthermore, metal particles in the center and carbon particles are large, although carbon deposits on the surface of metal at the center of on-ion-like structure in such a manner that carbon covers the surface of the metal.
Thus it is not possible to obtain with carbon materials disclosed in the above patent publications carbon electrodes that can be practically used. Consequently it is difficult to obtain a satisfactory capacity with nonaqueous secondary batteries.
Accordingly the present invention is intended to overcome the above described unfavorable conditions.